1. Field of the Invention
The present invention relates to semiconductor wafer testing.
2. Description of Related Art
In multi-level metallizations in integrated circuits, an interlayer dielectric (ILD) electrically isolates adjacent metal layers. The permittivity (∈) or dielectric constant (k) of the ILD is, therefore, an increasingly critical parameter as device dimensions shrink. By) way of background, the permittivity (∈) of a material is equal to the product of the material's dielectric constant (k) times the permittivity (∈0) of free space, i.e., ∈=(k) (∈0).
Second generation low permittivity, or low-k, dielectric materials, synthesized by chemical vapor deposition or spin-casting techniques, are currently being developed. These low permittivity dielectrics include both inorganic (silicon-based) and organic (carbon-based) materials, often with controlled levels of porosity. The use of a dielectric layer having a lower permittivity than previous generations of semiconductor devices reduces RC time delays, prevents cross talk and reduces power consumption in devices by decreasing the parasitic capacitance of the ILD layer.
Capacitance-voltage (CV) measurements on dielectric materials to determine permittivity are often made with mercury probes. There are several disadvantages with utilizing mercury probes. First, a minimum thickness of dielectric material is required for measuring materials with high permittivities. Second, new low permittivity materials (dielectric constants<3.9 (SiO2)) are often porous, whereupon the mercury contact area is affected. Third, new requirements for semiconductor fabrication facilities increasingly ban the use of mercury.
Similar to CV measurements, charge-voltage measurements on dielectric materials to determine the permittivity thereof have been recently made utilizing corona-based methods. These methods are based on a deposition of charge onto the dielectric surface with a subsequent surface voltage measurement. Disadvantages of the corona-based method include the time needed to make each measurement, the large measurement spot size, and poor likelihood that end users will allow for corona deposition of product wafers.
CV measurements on dielectric materials utilizing conductive elastic probes have been successful for measuring high permittivity, or high-k, materials down to a thickness of less than 1.0 nm. However, the current design of these conductive elastic probes developed for high permittivity materials cannot be used for measurement of low permittivity materials. This is due to the small contact diameter, e.g., between 30 μm and 50 μm, of the conductive elastic probe in combination with a thicker, e.g., >200 μm thick, low permittivity film resulting in a weak electrical signal of only several pF. Moreover, according to Hertzian calculations, the maximum pressure the prior art conductive elastic probes can apply may cause yielding of many organic low permittivity films.
Therefore, what is needed is a method and apparatus for determining the permittivity or dielectric constant, of the thicker, low permittivity dielectric layers on semiconductor wafers.